The custom is as fleeting as it is elusive. In a matter of a split second, the entire exchange is over, and the people who witnessed the covert operation often never even knew it happened. A challenge coin, tucked in the palm of the United States president, is passed to a member of the military in a seemingly ordinary handshake. For photojournalists who cover the president, immortalizing the exchange that takes place during that 'secret handshake' in a still image is something of a white whale. It's exceedingly rare that clear photos of such a delivery are captured. Yet, just two weeks ago, the feat was pulled off twice -- on consecutive days.

President Obama paid a three-day visit to California, and on his way out to the West Coast, he stopped in Arkansas to meet with families there whose lives were devastated by a recent outbreak of deadly tornadoes. Moments after the president touched down at Little Rock Air Force Base, veteran Associated Press photographer Susan Walsh managed to snap a photo of President Obama making the hand off to Col. Patrick Rhatigan, commander of the 19th Airlift Wing.

'This was the first time I successfully made a photo of the president exchanging a coin,' Walsh, a photojournalist with the AP for 23 years, told AOL.com. She was a member of the AP team that won a Pulitzer for its coverage of the Clinton-Lewinsky scandal.

On the day Rhatigan was given a coin, the handshake wasn't such a well-kept secret. 'I knew that the coin shake was going to happen,' she revealed. 'You never know if you're going to get it. It is kind of like photographing somebody winking. For me,' Walsh continued, 'making a picture of the coin exchange is just like photographing a magician when you can see how they do their trick. It is just very rare and cool to know that you made that photo.'

In a 2012 blog post, Reuters photographer Larry Downing describes the exchange as a 'secret handshake' not unlike 'the practice of slipping the folded $20 dollar bill to the maitre d' on date night.' Downing goes on to note that photographers find it 'nearly impossible to catch a coin in mid-flight and not many can claim those bragging rights.' Thus, even though she knew the handoff was coming, it's no small accomplishment for Walsh to have nailed that shot.

A day later, just after Obama arrived at the Air Station Miramar, near San Diego, he was caught in the act again, this time by AFP / Getty photographer Brendan Smialowski. The lucky recipient there was Sgt. Maj. Richard Charron, a longtime U.S. Marine originally from Lodi, N.J.

It's undoubtedly a thrill to photograph the president making his delivery of the coin, but what's it like to be on the receiving end of that exchange?

'It felt good to meet the commander in chief,' Charron told AOL.com in an email. He added that the gift from Obama came as a total surprise, though meeting a president was nothing new for him. 'Receiving the coin was a benny. During my career I've actually had the privilege of meeting four other presidents -- George H.W. Bush, George W. Bush, Bill Clinton and Ronald Reagan.' Charron says Obama had a brief message for him as he made the exchange during the secret handshake: 'Thank you for all of your hard work.' The challenge coin was the first Charron had received from a president, a new highlight in a collection of approximately 250 coins he's amassed over his long military career.

Not all challenge coin exchanges have gone smoothly for President Obama. In 2012, Obama attempted to hand a coin off to Sgt. Kristie Ness just as the president was about to board the Marine One helicopter. Ness awkwardly dropped the delivery of the coin, and, as shown in the slide show above, the president was memorably photographed from two angles -- by Jewel Samad of AFP / Getty and Jose Luis Magana of the Associated Press -- bending down to pick the coin up off the ground. He put the coin back in her hand and the two were photographed enjoying a laugh about the botched handoff.

Of his handoff, Charron reported that no such interference troubled the coin delivery Obama made to him, and he noted the swiftness with which the exchange was carried out. 'It actually happened so quick, it was just like a normal handshake,' Charron said.

The ritual of handing out challenge coins is a long tradition among those in the military. In addition to the presidential challenge coin -- which is the most coveted -- high-ranking officials and even lawmakers have been know to distribute them, largely to military personnel, according to Downing's blog post. In addition, Presidents Clinton, Bush and Obama have also given them to foreign dignitaries and diplomats as a symbol of welcoming and honorary membership. And often, President Obama leaves challenge coins on the graves of fallen U.S. soldiers.

If all of the above leaves you wanting a challenge coin for yourself, fear not. One doesn't necessarily have to be a soldier or diplomat to obtain one -- souvenir coins are also available online for purchase in the White House gift shop, ranging in price from $3.50 to $21.95.